Inkjet technology, which is well known for use in printing images onto paper, has also been employed in the fabrication of printed circuits. More particularly, inkjet printheads utilizing either thermal inkjet (TIJ) or piezoelectric (PZT) transducers have been employed to directly print circuit components onto circuit substrates.
There are two common manners in which fluid may be jetted from the printheads. One manner is known as drop-on-demand (DOD) and the other manner is known as continuous ink-jet (CIJ). As the name suggests, DOD systems include systems designed to eject fluid through application of a voltage through a PZT element or a thin film resistor element when a drop is desired. CIJ systems, on the other hand, include systems designed to jet a stream of fluid which break up into droplets due to Rayleigh instability. In comparing these types of systems, DOD systems are typically less complicated than CIJ systems, but DOD print heads have more complex fluidic design concerns and the DOD systems typically require three or more orders of magnitude greater energy to produce a droplet than CIJ systems because the fluid flow direction reverses during drop formation and break off. In addition, CIJ systems are typically less susceptible to nozzle imperfections than DOD systems. Therefore, manufacturing tolerances for nozzles implemented in DOD systems are typically much lower than those for nozzles implemented in CIJ systems. One result is that the costs associated with producing the CIJ system nozzles are oftentimes lower than those costs associated with producing the DOD system nozzles.
TIJ type printheads typically include a thin-film resistor for applying heat to superheat a small portion of the printing material near the heater element. During super-heating, a bubble of vaporized material is created and the explosive growth of the bubble expels the fluid in the nozzle of the printhead beyond the orifice to form droplets. The thin-film resistor is therefore positioned near the nozzle and the printing material is fed to a location between the thin-film resistor and the nozzle opening. The use of TIJ type printheads is substantially limited by the types of materials that may be ejected from the printheads. For instance, TIJ type printheads may be unable to supply sufficient heat power to super-heat high melting temperature, thermally conductive materials. Another drawback associated with the use of TIJ type printheads in printing circuit components is that its drop frequency is limited by the ac impedance associated with its fluidic circuit because it employs drop-on-demand type ac operation.
PZT, or lead-zirconate-titanate, type printheads typically include a transducer component whose volume changes when it receives a voltage pulse. This volumetric change generates a pressure wave (acoustic wave) that propels the printing material through a nozzle of the printhead. PZT type printheads are not suitable for use in the typically high temperature environments necessary for application of electrically conductive materials, e.g., metals, inorganic semiconductors and ceramics because PZT elements commonly used as the actuators are limited by the depolarization temperatures for their ferroelectricity.